1. Agree with gambino, never bring it up.
2. Don't do "commitment" things, no whole weekend dates, meeting family, friends, group dates with friends, company parties, their or your housework or DIY projects. Get together, have fun for a few hours or even most of a day, then do it again next week.
3. Never "help" them when they start up with it and are stumbling and floating the BS open ended "what are we?" questions. Expect them to speak and form ideas like adults, not weewee children. Answer bad questions with questions that expect a definitive answer. "What are we? well what do you want us to be?" when they give their lame answer and turn it back on you, "well I want to keep having the same good times we've been having lately!" Keep deflecting until a definite, reasonable request comes out of their mouths, "I want us to be exclusive, I want us to see only each other, etc"
4. Always use their exclusivity requests to get some firm groundrules out there. "What does just dating each other mean to you?" then listen, then "well what it means to me is we both make changes in our lives including XYZ."
For me personally the XYZ are 1. we don't go out to bars often or complain if we aren't "going out" more than one and infrequently two times a week. We very rarely go out where to parties or where there will be alcohol without each other 2. All drinking buddy orbiter friends of the opposite sex, all remotely recent exes, all pseudo friends who aren't or won't become friends of the relationship near instantly MUST GO, no exceptions. Only "real friends" not attention supply are around. 3. We never flirt with opposite sex no matter who is around or not, we don't keep ongoing flirtations at work. We let any work people who call at night "to discuss the project... but really to flirt" to talk about "the big deal" at work. We tell people unequivocally that we are in a relationship and firmly rebuff advances. We don't ever say "aw, he was just being friendly."
The XYZ for you may be totally different, but use their exclusivity requests as opportunities, if you are going to give them something they want, they are going to give you what you want. Make that crystal clear such that they can't rationalize or squirm around later. They will rationalize and squirm anyway, you will just have a pat response to repeat over and over, "remember when we had our exclusive talk? I was not joking around."